1. Field of the Invention
This invention generally relates to liquid crystal display (LCD) fabrication and, more particularly, to a structure and method for supporting a flexible LCD substrate during manufacturing.
2. Description of the Related Art
Thin film transistor (TFT) LCD displays are widely used with notebook type personal computers (PCs) due to their light weight and thin silhouette. More recently, TFT LCDs have been adapted for with personal digital assistants (PDAs), cellular telephones, and handheld game machines. These new applications demand less costly displays, while desktop PC applications demand higher image quality. TFT LCD displays are needed with enough diversification to satisfy both these contradictory requirements. For instance, poly-Si (polycrystalline silicon) TFT LCDs are suitable for the monitor of a notebook PC, because of their high image quality. Amorphous Si TFT LCDs are more suitable for use as a flat panel TV monitor, due to their lower cost. For the mobile devices, a flexible TFT LCD would be best type of display, as it can be made lighter and thinner, and less susceptible to breakable.
A number of researchers are attempting to realize more practical TFT LCDs on a plastic substrate. However, there are a number of problems that prevent the realization of a practical plastic TFT LCD display. Due to its flexibility and thinness, it is hard to securely hold a plastic substrate during the fabrication process. To overcome this issue, glue, or an adhesive have been used to attach the plastic substrate to glass. This method is suitable to fabricate plastic TFT LCD displays using conventional TFT LCD factory equipment and processes designed for use with a glass substrate LCD. To some extent, this method has been able to suppress the expansion of plastic substrate in response to heat or water absorption. However, if any air or water remains between the plastic and glass substrate, the air expands in the vacuum process and subsequently deposited films are deposited on the plastic in its expanded state. After the substrate is returned to normal atmospheric pressure, the plastic contracts and the overlying film can become cracked. To improve the plastic substrate LCD process, all the air and water between the plastic substrate and the glass support member must be removed when the plastic is attached to the glass.
As noted in U.S. Pat. No. 6,214,460 (Bluem et al.), screen printing of adhesives is known in the art and is used advantageously to apply adhesives to selected areas on a substrate. The adhesive printed or coated areas can subsequently be used to adhere to a second substrate. Typical screen-printable adhesives are pressure-sensitive adhesives which are tacky at room temperature, or heat-activatable adhesives, which are not tacky at room temperature, but become tacky when heated. Examples of screen-printable adhesives include (meth)acrylic polymers and copolymers dispersed in an organic solvent or water.
Acrylic adhesives, both pressure-sensitive and heat-activatable types, are widely used in industry because they are stable over time, and they can be formulated to adhere to a wide variety of different surfaces. With the advent of more stringent environmental controls, the technology in adhesives in general has evolved from solvent-based materials to water-based materials, and to a degree, solvent-free materials. Solvent-free acrylate adhesives are known and fall in various categories of processing such as heat-activatable coating and radiation curing which includes E-beam curing, ultraviolet light processing, and gamma radiation processing. Solvent-free crosslinked compositions are known in the art, but they provide little utility for adhesively bonding to other substrates since they are highly crosslinked and do not flow or become tacky on heating. Ultraviolet light processed adhesives are also used. While known adhesives processed by ultraviolet light have their own utility and advantages, they do not screen print well because they tend to become stringy during screen printing.
As noted in U.S. Pat. No. 5,699,139 (Aastuen et al.), thermal or barometric variations can affect LCD performance. The LC material in the display must fill the region between the two substrates perfectly, and the variation in the spacing of the two layers must be tightly controlled. As the LCD heats up due to either absorption of light energy or by ambient conditions, the pressure within the cell begins to build. Alternatively, the internal pressure may change due to ambient barometric pressure variations which must also be accounted for. The thermal expansion of the LC material and the thermal expansion of the substrates enclosing this material may not match, creating an internal pressure increase with rising temperature. If the substrate material is plastic (and therefore somewhat flexible), the portions of the substrate between the separation spacers can bow, changing the separation between the substrates. If the pressure variation becomes too great, the bonding of the separation spacers or the edge sealing can be compromised, and the LC cell can delaminate. Conversely, if the temperature or barometric pressure is lowered, a partial vacuum can be created in this region, creating bubbles within the LC material that may interfere with the display of information or otherwise damage the display. If cracks are formed in the adhesive during fabrication of LCD, this pressure problem is further accentuated.
FIGS. 1a through 1e are partial cross-sectional views of a flexible LCD substrate 10 during stages of fabrication (prior art). A layer of adhesive 12 binds a support substrate 14 to a flexible substrate 16. Reference designator 18 is an area in the adhesive that contains a bubble of water, air, or some other gas or liquid.
In FIG. 1b a vacuum has been created as a result of some LCD fabrication process, and the bubble 18 has expanded. The flexible substrate 16 does not lie flat.
In FIG. 1c an integrated circuit (IC) film 20 has been deposited overlying the flexible substrate 16. The IC film 20 can be a base coat layer of silicon dioxide, for example, of a thin film of silicon. Since the underlying flexible substrate 16 is not flat, the IC film 20 does not lie flat of the support substrate 14.
In FIG. 1d the LCD substrate 10 is returned to normal atmosphere. There is air pressure acting on the bubble region 18, that has expanded in the vacuum of the previous fabrication process.
In FIG. 1e the IC film has cracked as a result of the air pressure acting on the bubble region. If the IC film 20 had been a base coat, for example, the cracks in the film will permit impurities from the support substrate to migrate into overlying areas, such as into the active regions of transistors. It should be understood that the cracks may form as a result of several vacuum or annealing process cycles. It should also be understood that cracks may likewise form in IC films several layers above (not shown) the support substrate 14.
It would be advantageous if a flexible plastic LCD substrate could be held completely rigid during fabrication to improve the mechanical and electrical characteristics of the final product.
It would be advantageous if the adhesive used to hold a flexible LCD substrate during fabrication could be more evenly distributed across the glass substrate.
It would be advantageous if the integrated circuit (IC) films overlying the flexible LCD substrate could be formed with a more uniform thickness, without cracks or weak areas. It would likewise be advantageous if the flexible LCD substrate could be adhered to remain flat during the LCD fabrication procedures, to promote the formation of more uniformly thick overlying IC films.